A variety of two-piece induction seals have been developed. Seal products have application in the closure industry. The seal generally includes a compressing agent (e.g., a thickness of pulpboard or layers of synthetic foam) and an induction membrane layer (e.g., foil), with a wax layer between them to keep them in place during processing. The membrane layer further has an adhesive layer on its bottom surface which is generally a heat-activated adhesive layer. During bottle closure operations, the seal product is placed between the rim or other opening of the filled container and the cap. When energy is applied, the induction membrane layer becomes heated, thereby melting the wax and activating the adhesive. The result is the conversion of the one-piece unit into two pieces, with the adhesive layer bonding the membrane layer to the rim, and the melted wax being absorbed by paper, the compressing agent or an absorbing synthetic polymer therewith. The compressing agent generally remains lodged in the inner portion of the cap or other closure device.
In common application, the compressing agent is a pulpboard material. This organic material is suitable for absorbing the melted wax. However, this system presents numerous disadvantages. The pulpboard becomes a source of paper dust which can contaminate the contents of the container. In another alternative, the foil layer is covered with a paper layer. A wax layer initially binds the compressing agent to the paper layer of the foil. When energy is applied to the unit, the wax melts and is absorbed by the paper on the foil layer, rather than being absorbed into the synthetic foam compressing agent. Eventually, this can cause the paper layer to seal to the synthetic foam. Additionally, pulpboard or paper are moisture sensitive and can become distorted and altered by fluctuations in humidity levels. Moreover the wax-filled pulpboard can also serve as a growth medium for bacteria and other biological contaminants. Alternative seal structures have been developed to attempt to overcome these disadvantages. In one such alternative, the compressing agent is made of a synthetic foam material which is initially bound to a foil layer by a starch layer. Application of energy heats and transforms the starch layer, breaking the bond between the foam and the foil.
In still another alternative, the wax or starch layer is replaced by a pressure-sensitive adhesive. This adhesive effectively binds the compressing agent, be it pulpboard or synthetic foam, to the foil layer. The process of opening the cap imparts a shearing force which breaks that bond allowing the container to be opened. A principle disadvantage of this device is that the adhesive layer which is present on the surface of the compressing agent remains tacky. As a result, materials, such as pills or other contents of the container, dirt, and other debris, can become affixed to the inner surface of the cap. The prior art does not solve the traditional problems of contamination, because the paper layer on the foil can continue to serve as a biological growth medium. In addition, the paper layer can present structural issues by delaminating from the foil layer and by expanding and contracting due to changes in humidity. The starch residue remaining on the synthetic foam can continue to serve as a bacterial growth medium.
Unipac Corporation has developed a two-piece induction seal which uses as the compressing agent a synthetic foam material with a synthetic polymer underlayer made of TYVEK™ from DuPont. This seal has been found to solve some of the problems described above, but the TYVEK™ synthetic polymer does not present a uniform absorbing surface due to porosity dimensional instability. The limitations of TYVEK™ can be attributed to the inconsistent fiber composition related to the flash spinning manufacture method used in its production that results in long fiber content. As a result, wax residues remain on the surface of the TYVEK™ layer after induction sealing causing variable behavior. In some instances the TYVEK™ layer melts, creating difficulty in opening the container.
A further developed two-piece induction seal, described in U.S. Pat. No. 6,131,754 to Smelko for “Synthetic two-piece induction seal” issued Oct. 17, 2000, uses a synthetic foam layer material as the compressing agent with a laminated layer of TESLIN™ synthetic polymer underlayer made of an absorbing synthetic polymer. However, this prior art design was not commercialized due to the inability to maintain adequate adhesion after being wax laminated. TESLIN™ is composed of a very high molecular weight polyolefin phase and a filler phase that is primarily silica. During manufacturing of TESLIN™ mineral oil is used to incorporate the silica into the matrix of the polyolefin. This process gives the TESLIN™ the porosity that is an integral part of the films design. Unfortunately a small amount of residual mineral oil remains in the film's matrix after processing. It was determined that the mineral oil would migrate out of the film and dissolve the microcrystalline wax used to laminate the 2 piece structure resulting in premature separation of the laminate. The compatibility of TESLIN™ with solvents and reagents reflects its dual composition of polyolefin and silica. Bases with a pH level of less than approximately 8.5 have little effect on the dimensions of TESLIN™. Alkali bases (e.g. sodium or potassium hydroxide) at higher pH levels or elevated temperatures will attack the silica filler and lead to shrinkage as the silica is removed from the sheet. Elevated temperatures may also lead to dimensional changes with weaker bases, which is of concern in a variety of end uses. For example alkali bleach that is typically pH 9.5 and above would be considered a typical package requirement for secondary sealing, as produced in the design, and would be of concern due to the described dimensional instability issues.